Session-namespace commit-message temp files so git commit -F cannot consume a concurrent session's message

Goal

Give repo-root commit-message temp files a session-namespaced path convention, so a git commit -F <file> cannot silently consume a concurrent session's commit message.

The gap

guard_root_code_commit.sh and guard_piped_git_push.sh both steer agents to git commit -F <file> (the sanctioned form when message prose would trip the git-verb guard, per CLAUDE.md § Task Workflow API and #1722/#1756). Neither the guards' remediation text nor CLAUDE.md states a naming convention for that file, so agents reach for obvious un-namespaced paths like /tmp/fig_commit_msg.txt or /tmp/commit_msg.txt. /tmp is shared across ~15 concurrent fleet sessions.

-F on a foreign-but-existing file CANNOT fail loud: git reads whatever is there. The result is a commit whose content is correct and whose message belongs to a different task — undetectable from the commit's own exit code, and unfixable after the push (a shared-root history rewrite needs a force-push, banned by .claude/rules/auto-continuation.md STATE-TO-blocked criterion 2).

Two mechanisms make the collision easy to hit rather than exotic:

1. A PreToolUse guard block kills the ENTIRE compound, so a cat > /tmp/msg.txt <<EOF ... && git commit -F /tmp/msg.txt whose commit arm is blocked never writes the file. The retry then finds a stale or foreign file at that path and succeeds against it.
2. Guard-blocked commits are COMMON on this surface (uncertified payload, foreign staged index, piped-push), so the write-then-blocked-then-retry sequence is the normal path, not an edge case.

Precedent for the fix shape

The inline-payload path already has exactly this convention: #1948 made inline_lint_gate.py REFUSE the bare issue-keyed /tmp/issue-<N>-inline-payload.txt and require a <round-slug> making it round-unique, for the identical reason (concurrent same-issue rounds clobber a shared path). The commit-message path has no equivalent.

Proposed change

1. State a convention in CLAUDE.md § Concurrent repo-root committers and in both guards' remediation text: commit-message files go to /tmp/eps-<issue>-<session-slug>-<round-slug>-commit-msg.txt, never a bare /tmp/<something>_msg.txt.
2. Consider a guard arm that WARNs (not blocks) when -F <path> names a file whose mtime is older than the current turn or whose path lacks a session-unique component — the cheap detection is "the file existed before this session started".
3. Consider having the guards' remediation text emit a ready-to-paste namespaced path rather than a generic <msgfile> placeholder, since agents copy that text literally.

Evidence

origin/main commit 7cb8f68a2a — subject task #1336: full forward stage-transfer lattice figures (all 10 pairs), content is #2054's Results 2 Plot 2 figure round (7 files, verified byte-identical to the intended payload). Durable record: epm:progress note on #2054, 2026-08-25.

Not hypothetical and not recoverable after the fact — which is what makes the convention worth pinning rather than leaving to agent judgment.